280 Causes and Course of Organic Evolution 



jugation, and the origin of the meganucleus from the latter, 

 all suggest that here, as amongst the Htemoflagellata, the 

 chromatin or cognitic substance early separates after fertili- 

 zation into a trophonuclear and a kinetonuclear portion. 



As to details of the conjugation process Calkins (105: 156) 

 says for Paramoecium aurelia: "At the outset the two organ- 

 isms are more loosely attached, so that forceful ejection from 

 a pipette is sufficient to separate them. After twelve hours 

 union, however, the attachment is so firm that no amount 

 of force ^\i^\ break them apart without killing one or both. 

 The normal course of conjugation requires from eighteen to 

 thirty hours, according to the temperature, and during the 

 process the micronucleus of each cell divides twice; one of 

 the four micronuclei then divides again into dimorphic nuclei. 

 One of these nuclei is smaller than the other and acts as a 

 spermatic or wandering nucleus, wliile the other remains in 

 the parent cell. Each cell receives a wandering smaller micro- 

 nucleus from the other organism; this fuses with the larger 

 micronucleus to form the fertilization nucleus of the new indi- 

 viduals. Each fertilization nucleus then divides three times 

 in quick succession, and eight micronuclei are formed. P'our 

 of these begin to swell and to metamorphose into four new 

 macronuclei, while four remain as micronuclei. In the mean- 

 time the two conjugating cells separate, soon after the inter- 

 change of micronuclei, and the processes of reorganization 

 are carried out independently. The old macronucleus begins 

 to disintegrate by first forming a skein-like structure, and then 

 breaking down into granules which are finally absorbed by 

 the cell protoplasm. The process of reorganization requires 

 from one to three days before the first division of the fertilized 

 cell, which, as we have seen, contains four micronuclei and 

 four macronuclei. Each daughter cell, after the first di^^sion, 

 contains two macronuclei and two micronuclei, and the normal 

 nuclear relations are not reestablished until after the second 

 division, when the resultant four cells have each one macro- 

 nucleus and one micronucleus." 



Amongst the Infusoria the number of divisions previous 

 to reduction in and extrusion of chromatin substance has 



